The International Space Station generally operates in orbits at altitudes between 330-435 km above the Earth's surface. You have probably seen astronauts float about "weightless" up there in "zero-g."
1) If the Earth's gravity was zero at that altitude, what does Newton's First Law tell you about the shuttle's movement?
2) What is the ratio of the acceleration due to the Earth's gravity g on the surface of the Earth to g at 400 km above the Earth (RE = 6400 km).